Pointers and Arithmetic Workshop

Retrieval Prompts

1. State the rule for what p + n means when p has type T *.
2. Explain what & and * do, in one sentence each.
3. Write from memory the difference between int *a[10] and int (*a)[10].
4. State what p[i] is defined to mean in terms of * and +.
5. Explain when comparing two pointers with < is undefined.

Compare and Distinguish

Separate these pairs clearly:

• address (&x) versus value (*p)
• pointer to array (int (*)[10]) versus array of pointers (int *[10])
• T * versus T **
• function pointer R (*f)(args) versus function designator R f(args)

Common Mistake Check

Identify the error in each line:

1. char *s = "hello"; s[0] = 'H';
2. int *p; *p = 10;
3. int *f(void) { int x = 5; return &x; }
4. int a[5]; memset(a, 0, 5);
5. int a[5], *p = a; if (p + 10 < p) { ... }

Mini Application

For each snippet, first draw a memory diagram, then predict the output, then compile with gcc -Wall -Wextra -O1:

1. int a[4] = {10, 20, 30, 40};
   int *p = a + 2;
   printf("%d %d %d\n", *p, p[-1], p[1]);

2. int x = 5, y = 7;
   int *arr[] = { &x, &y };
   (*arr[0])++;
   printf("%d %d\n", x, y);

3. int x = 42, *p = &x, **pp = &p;
   **pp = 100;
   printf("%d\n", x);

4. int add(int a, int b) { return a + b; }
   int sub(int a, int b) { return a - b; }
   int (*fs[2])(int, int) = { add, sub };
   printf("%d\n", fs[1](10, 3));

5. Tricky cast prediction:

   uint32_t x = 0x11223344;
   unsigned char *p = (unsigned char *)&x;
   printf("%02x%02x%02x%02x\n", p[0], p[1], p[2], p[3]);

   Predict on a little-endian machine. Is the hex string 11223344 or 44332211?

Lab Tasks

• Implement my_strlen, my_strcpy, and my_strcmp using pointer arithmetic only (no array subscripting). Verify against strlen, strcpy, strcmp on 20 inputs.
• Build a tiny "shell of function pointers": a table mapping command strings to void (*)(int, char **) handlers; parse the command, dispatch, and print a help message if the command is unknown.
• Use a debugger: gdb ./program, set a breakpoint, print *p, print p, print &p. Get comfortable seeing pointer values and dereferencing them from the debugger.

Evidence Check

This page is complete only if, given a short snippet of pointer code, you can:

• draw a correct memory diagram before running the code
• predict the value of every *p-style expression
• explain the type of every declaration out loud